The present disclosure relates to a coating system for coating a part, such as a turbine engine component, and to a process for coating the part.
During EB-PVD ceramic coating, gas turbine blades and vanes are placed into fixture cans to protect under platform surfaces from being coated. The fixture cans are moved spatially and rotated through the vapor cloud of coating material to deposit coating thicknesses on airfoil and platform surfaces. Tumblers and rake arms are commonly used to provide these motions.
In the tumbler approach, the longitudinal axis of the airfoil is maintained at a roughly constant angle relative to the vertical axis of the coater. The part rotates about its longitudinal axis and the longitudinal axis is translated around a circular path. Translation occurs at a rate of 1 to 30 RPM, and the rotation around the longitudinal axis occurs 4-10× slower.
In the rake approach, the longitudinal axis of the airfoil is swept over a range of angles relative to the vertical axis of the coater. The range of angles of the sweep is centered on the horizontal axis of the coater. The range of tilts is typically between −30 degrees and +30 degrees, but different tilting schemes are used for different parts. For example, vane tilting is often different from blade tilting. Independent of the tilting program, the part is rotated around its longitudinal axis at 10 to 40 RPM. The rotation rate of the airfoil in the tumbler approach is 5 to 10× slower than in the rake approach.
FIG. 1 illustrates a cross section of an unmasked part. EB-PVD is a line of sight process. The thickness and microstructure of the ceramic coating varies on different part surfaces due to the part geometry. The amount and average incidence angle of vapor arriving onto the part over a full period of manipulation varies for different part surfaces. The zones labeled in FIG. 1 roughly demarcate areas over which thickness and microstructure are roughly constant. However, transitions from zone to zone are relatively smooth, not abrupt. The abruptness of the transition is a function of the change in the radius of curvature of the surface.
It should be noted that the foregoing discussion is only valid at the midspan location of the blade, since as you move along the surface toward the platform, the platform begins to mask the vapor flux as well. Larger platforms, such as on vanes, results in more masking. The above discussion also ignores any features on the part fixtures or part manipulators that may get interposed between the part and the vapor source during the part manipulation during coating.
Zone 1 of the part receives high vapor flux, since no surface of the part gets interposed between the vapor source and this area during part manipulation. This area also gets closer to the vapor source relative to zone 2. Vapor incidence angle distribution is predominantly symmetrical and centered on the surface normal. This results in high thickness and optimal microstructure.
Zone 2 of the part receives high vapor flux, since no surface of the part gets interposed between the vapor source and this area during part manipulation. Vapor incidence angle distribution is predominantly symmetrical and centered on the surface normal. This results in high thickness and optimal microstructure, but less thickness than Zone 1 because Zone 1 gets closer to the vapor source.
Zone 3 of the part is very similar to Zone 1, but smaller radius of curvature reduces thickness slightly, relative to Zone 1. This zone also gets close to the vapor source. Vapor incidence angle distribution is predominantly symmetrical and centered on the surface normal. A smaller convex radius of curvature enhances the microstructure by increasing column diameter and the width of gaps between columns, which enhances strain tolerance.
Zone 4 of the part is very similar to Zone 3, but smaller radius of curvature reduces thickness slightly, relative to Zone 3. This zone also gets close to the vapor source. Vapor incidence angle distribution is predominantly symmetrical and centered on the surface normal. Still smaller convex radius of curvature further enhances the microstructure by increasing column diameter and the width of gaps between columns which enhances strain tolerance.
The significant difference between zone 5 and zone 4 is that the trailing edge of the part gets interposed between the vapor source and this surface over a portion of every part manipulation period. Vapor incidence angle distribution is not symmetrical though still centered on surface normal. This results in less thickness and a slight tilt to the growth angle of the ceramic columns relative to the surface normal. Columns grow slightly tilted away from the trailing edge since less vapor comes from that direction.
The significant differences between zone 6 and zone 4 is that both the leading edge and the trailing edge of the part gets interposed between the vapor source and this surface over a portion of every part manipulation period, and the surface curvature is concave, not convex. Vapor incidence angle distribution is less broad, though still centered on surface normal. The total vapor flux per part manipulation period is reduced for the same reason. Thus, this zone typically is the thinnest on the part. Since the change in surface curvature is more abrupt toward the leading edge than the trailing edge, the vapor incidence angle distribution is not symmetrical. Columns grow significantly tilted away from the leading edge, since less vapor comes from that direction than from the leading edge. The narrowing of the vapor incidence angle and the concave radius of curvature results in smaller column diameters and narrower gaps between columns. As a result, this zone has the lowest strain tolerance on the part.
Zone 7 is similar to zone 6, but lower concave radius of curvature and more distance from the results in less of an effect on the thickness and on the microstructure. Columns grow slightly tilted away from the trailing edge, since the trailing edge is closer than the leading edge, so the effect of trailing edge shadowing is greater, less vapor arrives from that direction.
As can be seen from the foregoing discussion, although the coating process is line of sight from the ingot source to the part, the airfoil geometry affects more coating on the leading and trailing edges of the part because of arc angle exposure time. To improve the thickness ratios between other airfoil surfaces and the edges, the edges are commonly masked with various forms of shadow bars or shields used to collect some of the coating.
Referring now to FIG. 2, there is shown a cross section of a part perpendicular to the longitudinal axis of the airfoil and with a masked leading edge. In zone 1, there is a minor change to the unmasked part since the presence of the leading edge mask casts a bigger shadow on the zone during some portion of the period of part manipulation, such that the coating would be slightly thinner with slightly smaller column diameters, and slightly smaller column boundary widths.
In Zone 2, there is a very small change relative to the unmasked part.
In Zone 3a, there is a small change relative to the unmasked part. The presence of the leading edge mask casts a bigger shadow on this zone during some portion of the period of part manipulation, such that the coating would be slightly thinner with slightly smaller column diameters, and slightly smaller columns.
In Zone 3b, there is a significant change in thickness and microstructure relative to the unmasked mart. The presence of the mask reduces coating thickness, reduces column diameters and narrows column boundary widths. Also the direction of columnar growth tilts away from the leading edge mask, due to reduced vapor coming from the direction of the leading edge mask.
In Zone 4a, there is a significant change in thickness and microstructure relative to the unmasked part. Columns are tilted the same direction as in Zone 3b, but are tilted at a larger angle relative to surface normal, since vapor is only coming from the gap between the mask and the part. The severe angles of columnar growth dramatically reduce erosion resistance, and spallation resistance, but to a lesser effect for the latter.
In Zones 4b and 4c are very similar, except the columns in Zone 4c tilt in the other direction.
Zone 4d is very similar to Zone 4a, but the columns tilt in the other direction.
The difference between Zone 5a on the masked part to Zone 5 on the unmasked part is a further narrowing of the vapor incidence angle distribution due to the presence of the mask that reduces coating thickness, column diameters and column boundary widths. Columnar growth is also tilted further toward the trailing edge since there is even less flux coming from the direction of the leading edge. This reduces the variability of the coating in this zone.
Zone 5b has a similar effect as in Zone 5a, but to a lesser degree. There is much less effect on degree of tilt on columnar growth than in Zone 5a.
Zone 6 has a very small change relative to the unmasked part.
Zone 7 has a minor change relative to the unmasked part. The presence of the leading edge mask casts a bigger shadow on this zone during some portion of the period of part manipulation, such that the coating would be slightly thinner with slightly smaller column diameters, and slightly smaller column boundary widths.
Sometimes trailing edge masks are used in lieu of the leading edge masks. The same effects would apply—significant thickness and microstructure changes close to the mask, and lesser effects further away.
Since these masks are attached to the fixture cans and are stationary relative to the airfoil, the coating below the masks may have a columnar structure that is off vertical, dependent on the size of the mask and its distance from the airfoil surface. Columnar coating structure that is off vertical quickly loses its fracture toughness, going to about zero at 45 degrees.